First-past-the-post requires concentrated areas of support for large representation in the legislature whereas proportional representation better reflects the range of a population's views. Proportional systems may have multi-member districts with more than one representative elected from a given district to the same legislative body, and thus a greater number of viable parties. Duverger's law states that the number of viable political parties is one plus the number of seats in a district.

A system where only two parties have the possibility of winning an election is called two-party system. A system where only three parties have a realistic possibility of winning an election or forming a coalition is sometimes called a "Third-party system". But, in some cases the system is called a "Stalled Third-Party System," when there are three parties and all three parties win a large number of votes, but only two have a chance of winning an election. Usually this is because the electoral system penalises the third party, e.g. as in Canadian or UK politics. In the 2010 elections, the Liberal Democrats gained 23% of the total vote but won less than 10% of the seats due to the first-past-the-post electoral system. Despite this, they still had enough seats (and enough public support) to form coalitions with one of the two major parties, or to make deals in order to gain their support. An example is the Conservative-Liberal Democrat coalition formed after the 2010 general election. Another is the Lib-Lab pact during Prime Minister James Callaghan's Minority Labour Government; when Labour lost its three-seat majority in 1977, the pact fell short of a full coalition. In Canada, there are three major federal political parties; the Conservative Party of Canada, the Liberal Party of Canada, and the New Democratic Party of Canada. However, the Liberals and Conservatives have been the only two parties to form government in Canada with the New Democrats as the third party, except in the 2011 Canadian election when the New Democrats were the Official Opposition.

Unlike a one-party system (or a two-party system), a multi-party system encourages the general constituency to form multiple distinct, officially recognized groups, generally called political parties. Each party competes for votes from the enfranchised constituents (those allowed to vote). A multi-party system prevents the leadership of a single party from controlling a single legislative chamber without challenge.

If the government includes an elected Congress or Parliament, the parties may share power according to proportional representation or the first-past-the-post system. In proportional representation, each party wins a number of seats proportional to the number of votes it receives. In first-past-the-post, the electorate is divided into a number of districts, each of which selects one person to fill one seat by a plurality of the vote. First-past-the-post is not conducive to a proliferation of parties, and naturally gravitates toward a two-party system, in which only two parties have a real chance of electing their candidates to office. This gravitation is known as Duverger's law. Proportional representation, on the other hand, does not have this tendency, and allows multiple major parties to arise. But, recent coalition governments, such as that in the U.K., represent two-party systems rather than multi-party systems. This is regardless of the number of parties in government.

A two-party system requires voters to align themselves in large blocs, sometimes so large that they cannot agree on any overarching principles. Some theories argue that this allows centrists to gain control. On the other hand, if there are multiple major parties, each with less than a majority of the vote, the parties are strongly motivated to work together to form working governments. This also promotes centrism, as well as promoting coalition-building skills while discouraging polarization.[2][3]